2019-06-01_New_Scientist

14 | New Scientist | 1 June 2019

DNA detective work has revealed

the ancient origins of the

domesticated watermelon.

Its wild counterparts found

in parts of Africa are nothing like

domesticated varieties. They are

small, round and have white flesh

with a very bitter taste due to

compounds called cucurbitacins.

There has long been a debate

about when and where they

were domesticated, with some

suggesting it took place in south

or west Africa.

However, pictures on the walls

of at least three ancient Egyptian

tombs depict what look like

watermelons – including one

that looks strikingly like modern

varieties. And in the 19th century,

watermelon leaves were found

placed on a mummy in a tomb

dating back around 3500 years.

When Susanne Renner at the

University of Munich, Germany,

learned about these leaves, she

realised their DNA might reveal

what the ancient melons were like.

She also discovered that some of

the leaves had been sent to the

botanist Joseph Hooker, then

head of Kew Gardens in London.

Kew gave Renner’s team a

tiny sample of one leaf, and her

colleague Guillaume Chomicki,

now at the University of Oxford,

sequenced its DNA. The team were

only able to get a partial genome

sequence, but it includes two

crucial genes that reveal what

these melons were like. “We

were so lucky,” says Renner.

One of these genes controls

the production of the bitter

cucurbitacins. In the 3500-year-

old melon, there was a mutation

that disabled this gene, meaning it

had sweet flesh just like modern

varieties (bioRxiv, doi.org/c56t).

The other gene codes for an

enzyme that converts the red

pigment lycopene – the pigment

that makes tomatoes red – into

another substance. This gene

was also disabled by a mutation,

meaning lycopene accumulates

and the fruit would have red flesh.

What the team can’t tell from

the partial sequence is how large

the melons were and whether they

were elongated or spherical. But

one ancient Egyptian picture is of

what appears to be an elongated

melon, so it seems farmers had

bred watermelons at least 3500

years ago with most if not all of

the key features seen today.

The DNA also reveals that the

ancient melon was closely related

to a sweet watermelon with white

flesh still grown in the Darfur

region of Sudan. That suggests

the watermelon was first grown

by farmers in this region and

use of the plant then spread

northwards along the Nile, with

further changes like red flesh

occurring along the way. ❚

WHO is harming the ozone layer?

In recent decades, amounts of

chlorofluorocarbons (CFCs) in the

atmosphere – gases that damage

ozone and warm the planet – have

been declining, which is good news.

But the second most abundant

CFC, trichlorofluoromethane or

CFC-11, has seen levels rising since

2013, puzzling researchers when

they detected the increase last year.

The culprit appears to be illegal

production of CFCs in China,

in defiance of a ban on their

manufacture from 2010.

An international team traced the

source of the spikes in the pollutant

back from two monitoring stations,

one in Japan and one in South

Korea. To pinpoint which country

was responsible, the team used

models to simulate how the

atmosphere would distribute

plumes of CFC-11.

The results pointed clearly to

the eastern Chinese provinces of

Shandong and Hebei (Nature, doi.

org/c56q). “We think the rise from

this region explains the increase

in CFCs,” says Matt Rigby at the

University of Bristol, UK.

Globally, around 65,000 tonnes

of CFCs leak into the air each year.

This is usually gas that escapes

from materials used in buildings

and from refrigerators rather than

from gas production. The new

research suggests China is

responsible for 40 to 60 per cent of

the previously unexplained rise in

CFCs. China appears to have been

emitting 10,000 tonnes a year

between 2013 and 2017,

more than double its previous

contribution.

If this continues unchecked,

it poses serious risks for the

recovery of the ozone layer, as well

as adding to global warming.

“The extra emissions we’ve

seen over the last four to five

years, if they stopped tomorrow

it shouldn’t delay ozone recovery

substantially,” says Rigby. “But if

they continue, we could see delays

of several years, even decades.” ❚

Adam Vaughan

China confirmed as

source of ozone-

eating chemicals

Pollution

Plant domestication

Michael Le Page

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65,

Number of tonnes of CFCs that leak

into the atmosphere each year

Modern watermelons

(above) resemble some

tomb paintings (below)

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Ancient Egyptians enjoyed

sweet, red watermelons